Derive Macro packing_codegen::Packed

#[derive(Packed)]
{
    // Attributes available to this derive:
    #[packed]
    #[pkd]
}

Derive for Packed

Attributes

Struct level

Optional flags for packed attribute when used at the struct level:

Name	Description
little_endian	(default) sets the struct default endianness to little endian
big_endian	sets the struct default endianness to big endian
lsb0	(default) sets the struct bit ordering such that the least significant bit is bit 0 and the most significant bit is bit 7
msb0	sets the struct bit ordering such that the most significant bit is bit 0 and the least significant bit is 7

Field level

Optional parameters for packed attribute when used at the field level:

Name	Description	Default
start_byte	Zero based offset from the start of the struct where this field starts	Inferred from the end of the previous field
end_byte	Zero based offset from the start of the struct where this field ends (inclusive)	Inferred from start_byte + field width
start_bit	The bit where this field starts within the start byte. lsb0/msb0 flips the range of this field (7 to 0 vs 0 to 7)	7 for lsb0, 0 for msb0
end_bit	The bit where this field ends within the end byte (inclusive). lsb0/msb0 flips the range of this field (7 to 0 vs 0 to 7)	0 for lsb0, 7 for msb0
width	(partially tested) The width of the field in bits. This is checked against start/end byte/bit if they are specified	Inferred from start/end byte/bit and/or the native width of the field type
space	(partially tested) The space before the field in bits. Allows shifting a field along by a number of bits	0

Mandatory values for pkd attribute:

Index	Description
0	start_byte
1	end_byte
2	start_bit
3	end_bit

Note about width/space

The main use case of this macro is explicitly specifying start_byte, end_byte, start_bit and end_bit on every field. For SCSI and USB specifications, these values can be read off the tables provided more easily than trying to work out how many bits the field is total, adding offsets, etc, etc. I found transcribing these values directly from the spec much less error prone than attempting to do maths around offsets and trying to remember the exact algorithm the packing macro(s - I tried several) used.

However, this derive also supports inferring field alignments from the width of the field type, where the last field ended, etc. This is provided so projects can use it for the complex explicit case mentioned above but also use it for the more trivial alignments you'd expect from repr(C) or repr(Packed). width and space were added to allow the case where most of the struct is as you'd expect but a handful of fields are slightly different. This was working at the time of implementation but has no tests around it currently so may get broken. //TODO: add tests for all supported cases.